Bentonite, having the property of absorbing water and swelling, is being widely used as a grouting material for constructing a water-blocking wall for protecting a heat transfer layer of a underground heat exchanger in GSHP (Ground Source Heat Pump) systems. However, since pure bentonite has low thermal conductivity, it should be mixed with auxiliary component such as sand, alumina or graphite when using it in the GSHP systems where high heat transfer rate is required.
In case that bentonite is to be used as a grouting material, usually bentonite powder is mixed with water first, then said auxiliary component such as sand etc. is added, and resulting mixture in slurry form is used. However, when bentonite powder, water and the auxiliary component are mixed, since the materials have different specific gravities, separation of materials occurs over time, and it is hard to use the mixture.
In order to solve such a problem, Korean Patent No. 1,471,003 teaches a method for producing granulated bentonite-based grouting material, in which bentonite is mixed any of sand, alumina, or graphite, and the mixture is granulated. According to this method, at least one of sand, alumina and graphite is added to bentonite raw material as an auxiliary component, and the resulting mixture is heated, then the molded body of bentonite is pulverized by grinding to produce granules of uniform sizes.
However, in the course of using this patented method, sintering the materials at high temperature (700 to 1,200° C.) decreases the water-blocking capability due to the thermal deformation of bentonite, and producing costs are increased because of energy cost for heating and complicated processes such as cooling step and pulverizing step. In addition, when the ratio of auxiliary components such as sand increases in order to raise the thermal conductivity, excessive bentonite powder which has not been reacted with sand is dissolved in water as time goes on, or separation of bentonite and sand could happen.
The other problem is that, since preparing only the slurry, which is formed by mixing the granulated bentonite with water, is not sufficient to block water at the time of construction, slurry should be prepared by mixing bentonite powder and water first, then the granulated bentonite is to be added thereto, thereby, the construction procedure becomes inconvenient.